With the gradual depletion of hydrocarbon reserves found onshore, there has been considerable attention attracted to the drilling and production of oil and gas wells located in water. In relatively shallow water, wells may be drilled in the ocean floor from bottom founded, fixed platforms. Because of the large size of structure required to support drilling and production facilities in deeper and deeper water, bottom founded structures are limited to water depths of less than about 1000-1200 feet. In deeper water, floating drilling and production systems have been used in order to reduce the size, weight and cost of deep water drilling and production structures. Ship-shape drill ships and semi-submersible buoyant platforms are commonly used for such floating facilities.
When a floating facility is chosen for deep water use, motions of the vessel must be considered and, if possible, constrained or compensated for in order to provide a stable structure from which to carry on drilling and production operations. Rotational vessel motions of pitch, roll and yaw involve various rotational movements of the vessel around a particular vessel axis passing through the center of gravity. Thus, yaw motions result from a rotation of the vessel around a vertically oriented axis passing through the center of gravity. In a similar manner, for ship-shape vessels, roll results from rotation of the vessel around the longitudinal (fore and aft) axis passing through the center of gravity causing a side to side roll of the vessel and pitch results from rotation of the vessel around a lateral (side to side) axis passing through the center of gravity causing the bow and stern to move alternately up and down. With a symmetrical or substantially symmetrical platform such as a common semi-submersible, the horizontally oriented pitch and roll axes are essentially arbitrary and, for the purposes of this disclosure, such rotations about horizontal axes wil be referred to as pitch/roll motions.
All of the above vessel motions are considered only relative to the center of gravity of the vessel itself. In addition, translational platform motions must be considered which result in displacement of the entire vessel relative to a fixed point, such as a subsea well head. These motions are heave, surge and sway. Heave motions involve vertical translation of the vessel up and down relative to the globally fixed point along a vertically oriented axis passing through the center of gravity. For ship-shape vessels, surge motions involve horizontal translation of the vessel along a fore and aft oriented axis passing through the center of gravity. In a similar manner, sway motions involve the lateral, horizontal translation of the vessel along a left to right axis passing through the center of gravity. As with the horizontal rotational platform motions discussed above, the horizontal translational motions, surge and sway, in a symmetrical or substantially symmetrical vessel such as semi-submersible are essentially arbitrary and, in the context of this specification, all horizontal translational vessel motions will be referred to as surge/sway motions.
Combinations of the above-described motions encompass platform behavior as a rigid body in six degrees of freedom. The six components of motion result as responses to continually varying harmonic wave forces. These wave forces are first said to vary at the dominant frequencies of the wave train. Vessel responses in the six modes of freedom at frequencies corresponding to the primary periods characterizing the wave trains are termed "first order" motions. In addition, a variable wave train generates forces on the vessel at frequencies resulting from sums and differences of the primary wave frequencies. These are secondary forces and corresponding vessel responses are called "second order" motions.
A completely rigid structure fixed to the sea floor is completely restrained against response to the wave forces. An elastic structure, that is, elastically attached to the sea floor, will exhibit degrees of response that very according to the stiffness of the structure itself, and according to the stiffness of its attachment to the firmament at the sea floor. A "compliant" offshore structure is usually referred to as a structure that has low stiffness relative to one or more of the response modes that can be excited by first or second order wave forces.
Floating production or drilling vessels have essentially unrestricted response to first order wave forces. However, to maintain a relatively steady proximity to a point on the sea floor, they are compliantly restrained against large horizontal excursions by a passive spread cantenary anchor mooring system or by an active controlled-thruster dynamic positioning system. These positioning systems can also be used to prevent large, low frequency (i.e. second order) yawing responses.
While both ship-shaped vessels and conventional semi-submersibles are allowed to freely respond to first order wave forces, they do exhibit very different response characteristics. The semi-submersible designer is able to achieve considerably reduced motion response by: (1) properly distributing buoyant hull volume between columns and deeply submerged pontoon structures, (2) optimally arranging and separating surface-piercing stability columns and (3) properly distributing platform mass. Proven principles for these design tasks allow the designer to achieve a high degree of wave force cancellation such that motions can be effectively reduced over selected frequency ranges.
The design practices for optimizing semi-submersible dynamic performance depend primarily on wave force cancellation to limit heave. Pitch/roll responses are kept to acceptable levels by providing large separation distances between the corner stability columns while maintaining relatively long natural periods for the pitch/roll modes. This practice keeps the pitch/roll modal frequencies well away from the frequencies of first order wave excitation and is, thus, referred to as "detuning".
Another class of compliant floating structure is moored by a vertical tension leg mooring system. The tension leg mooring also provides compliant restraint of the second order horizontal motions. In addition, such a structure stiffly restrains vertical first and second order responses, heave and pitch/roll. This form of mooring restraint would be essentially impossible to apply to a conventional ship-shape monohull due to the wave force distribution and resultant response characteristics. Therefore, this vertical tension leg mooring system is generally conceived to apply to semi-submersible hull forms which can mitigate total resultant wave forces and responses to levels that can be effectively and safely constrained by stiffly elastic tension legs.
This type of floating facility, which has gained considerable attention recently, is the so-called tension leg platform (TLP). The vertical tension legs are located at or within the corner columns of the semi-submersible platform structure. The tension legs are maintained in tension at all times by insuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. When stiffly elastic continuous tension leg elements called tendons are attached between a rigid sea floor foundation and the corners of the floating hull, they effectively restrain vertical motions due to both heave and pitch/roll-inducing forces while there is compliant restraint of movements in the horizontal plane (surge/sway and yaw). Thus, a tension leg platform provides a very stable floating offshore structure for supporting equipment and carrying out functions related to oil production.
As water depth (and, thus tendon length) increases, tendons of a given material and cross-section become less stiff and less effective for restraining vertical motions. To maintain acceptable stiffness, the cross-sectional area must be increased in proportion to increasing water depth, thereby increasing the weight of the tendons and the size of the floating structure to maintain tension on the heavy tendons. For installations in deeper and deeper water, a tension leg platform must become larger and more complex in order to support a plurality of extremely long tension legs and/or the tension legs themselves must incorporate some type of buoyancy to reduce their weight relative to the floating structure. Such considerations add significantly to the cost of a deep water TLP installation.
In addition, in deeper and deeper water, a greater percentage of the hull displacement must be dedicated to excess buoyancy (i.e. tendon pretension) to restrict horizontal offset. Station-keeping is a key role for the mooring system. The vertical tension leg mooring system provides the capacity to hold position above a fixed point on the sea floor as any horizontal offset of the platform creates a horizontal restoring force component in the angular deflection of the tendon tension vector. In deeper and deeper water, it requires greater tendon pretension to provide enough restoring force to keep the TLP within acceptable offset limits. This increase leads to larger and larger minimum hull displacements. The use of a hybrid mooring system as described for this invention reduces the impact of increasing water depth on minimum hull displacement and tendon pretension.